This invention relates to a lock-in amplifier. More particularly, this invention relates to a novel form of lock-in amplifier which is a digital lock-in amplifier.
Lock-in amplifiers, also referred to as phase-sensitive detectors or synchronous amplifiers, are widely used in scientific laboratories to detect time periodic signals from a device or experiment that occurs with a definite phase relation to a reference signal. Previously, lock-in amplifiers were based on analog circuit techniques. However, the analog lock-in amplifiers inherently have a transient recovery time which is long compared to a period of the incoming signal. The long transient recovery times result from the use of exponential filters, such as cascaded and decoupled RC filters, which require about 10 periods to reach 99.9% of the final value. This can be quite long at low frequencies, indicating poor performance for analog lock-in amplifiers at frequencies of less than a few Hertz. Further, analog lock-in amplifiers provide no information regarding the harmonics of the time periodic signals and are sensitive to interfering frequencies at odd harmonics of the reference frequency employed. In addition, two complete analog lock-in amplifiers are required to simultaneously measure the time periodic signal response in phase and in quadrature with the reference frequency.
Digital lock-in amplifiers are viewed as a means to overcome some of the problems of analog lock-in amplifiers. The first digital lock-in amplifier was proposed by Cova et al, Review of Scientific Instruments, Vol. 50, page 296 (1979) in very simple form. It was not subject to some of the drawbacks of the analog lock-in amplifiers previously known and the output provided was in digital form for convenient numerical analysis or storage.
The skilled artisans would still prefer a digital lock-in amplifier which according to the present invention provides more information with the output of a digital lock-in amplifier and includes stabilizing and adjustment for the amplitude of the reference frequency used. Still further, the present invention contemplates a dual digital lock-in amplifier having one digital lock-in amplifier which measures two signals simultaneously and includes a feedback and control system for one of them. These and other advantages and objectives of the present invention are disclosed more fully hereinbelow.